Tamoxifen resistance is a major problem in the treatment of Estrogen Receptor (ER) positive patients. Our analysis of human breast cancer tissue samples indicated association of lower expression of Hexamethylene bis-acetamide (HMBA) Inducible Protein 1 (HEXIM1) with tumor recurrence in patients who received tamoxifen We also reported that HEXIM1 is required for inhibition of ER by tamoxifen; tamoxifen-liganded ER induced HEXIM1 recruitment to the promoter region of ER target genes and decreased the recruitment of positive transcription elongation factor b (P-TEFb) to the coding regions of these genes. As a consequence the phosphorylation of RNAP II at serine 2 (S2P RNAPII) on coding regions of ER target genes is inhibited. In support of our mechanistic studies, downregulation of HEXIM1 expression resulted in attenuation of the inhibitory effects of tamoxifen on estrogen-induced gene expression and proliferation. In mouse studies, downregulation of HEXIM1 expression resulted in mammary tumors, hormone independent mammary morphogenesis, and attenuation of the repressive effects of tamoxifen on estrogen-induced tumor growth. ChIP-seq analyses to further define the mechanism of action of HEXIM1 indicate enrichment for genes involved in the response to estrogens and histone modifications among the direct targets of HEXIM1. These data revealed a role for KDM5B (a demethylase of lysine 4 of histone 3, H3K4) as a critical mediator of HEXIM1 action. HEXIM1 induced recruitment of KDM5B, resulting in downregulation of H3K4me2 levels on ER target genes. As a consequence, FOXA1-induced recruitment of ER? is inhibited. Thus HEXIM1 inhibits ER? directly (by interacting with ER? and preventing its functional interaction with the transcriptional elongation machinery) or indirectly (by attenuating histone modifications required for recruitment of ER?). Both of these actions may be mediated by KDM5B, and supports a critical role for KDM5B in the inhibitory effects of tamoxifen. Further supporting a role in hormone resistance is that KDM5B is phosphorylated and inhibited by AKT, which upregulates ER activity and is associated with hormone resistance. However, HEXIM1 may counteract this novel activity of AKT through induction of an inhibitor of PI3K/AKT, p85?. Based on our results we hypothesize that HEXIM1 regulates a novel transcriptional network involving the histone modifier KDM5B that receives input from PI3K/AKT signaling, but critical for suppression of mammary tumorigenesis and hormone resistance. Our goals are to: (1) Further define the role of KDM5B in the regulation of hormone dependence and response to SERMs by HEXIM1 and determine how HEXIM1 upregulates gene transcription (2) Further delineate the cross-talk between AKT and HEXIM1/KDM5B in the regulation of tamoxifen resistance (3) Validate the role of HEXIM1 and HEXIM1 mediators in the regulation of hormone dependence and the response to SERMs in vivo.